German Patent Application No. DE 101 39 142 contains a description of an exhaust-gas treatment device of an internal combustion engine, in which an SCR (selective catalytic reduction) catalytic converter is employed to reduce the NOx emissions, which reduces the nitrogen oxides contained in the exhaust gas to nitrogen, using ammonia as the reducing agent. The ammonia is obtained from a solution of urea in water in a hydrolysis catalytic converter upstream from the SCR catalytic converter. The hydrolysis catalytic converter converts the urea contained in the aqueous urea solution to ammonia and carbon dioxide. To ensure exact metering, provision is made for determining the concentration of the aqueous urea solution. The aqueous urea solution is brought to a specified pressure using a pump. A metering valve sets a specified rate of flow. Pressurized air is mixed into the aqueous urea solution in a mixing chamber. The aqueous urea solution, together with the added air, is sprayed into the exhaust gas of the internal combustion engine in such a way that a largely uniform flow to the SCR catalytic converter is attained.
German Patent Application No. DE 10 2004 018 221 (not previously published) contains descriptions of a method and a device according to the species, in which a pressurized reagent is also sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter. The reagent pressure is set at a prescribed specified reagent pressure, depending on a characteristic value. An operating variable of the internal combustion engine and/or a characteristic of the exhaust gas of the internal combustion engine may be used as the characteristic. The prescribed specified reagent pressure is regulated under a regulation system in which the actual reagent pressure is detected by a reagent pressure sensor. Pressurized air may be added to the reagent. The pressure of the pressurized air may also be regulated under a regulation system to a prescribed specified pressurized air pressure depending on a characteristic, the actual pressure of the pressurized air being detected by a pressurized air pressure sensor. A defect in at least one of the pressure sensors may result in reduced performance of the SCR catalytic converter, with the consequence that uncleaned exhaust gas is able to escape into the environment.
German Patent Application No. DE 10 2004 044 506 (not previously published) also contains descriptions of a method and a device according to the species, in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter. The pressurized air is routed through a check valve, which has an opening pressure. There is provision for a diagnosis of the pressurized air pressure, which begins at a starting point when a pressurized air regulating valve is closed. At least at a second point in time a check is performed to determine whether the pressurized air pressure corresponds at least to a lower threshold value, which corresponds at least approximately to the sum of the ambient air pressure and the opening pressure of the check valve. An error signal is supplied if the condition is not fulfilled.
An object of the present invention is a method for operating an exhaust-gas treatment system of an internal combustion engine in which a reagent is introduced into the exhaust-gas area of the internal combustion engine, as well as a device for carrying out the method, which contribute to exact metering of the reagent.